While scuba diving, how cold should the water be for a drysuit to be a necessity?

I have many diving certifications, but they've all been taught and dove in tropical waters. I usually wear a 3mm wetsuit with a 3mm hood, gloves and booties in warmer waters just for comfort. I wanted to start getting in to freshwater diving and ice diving where I live in Canada. I'm going to be taking a Drysuit course within the next two months and was curious to know exactly between what temperatures should a person be advised to wear a dry suit? Or is a 7mm wet suit suitable for cooler (ice free, yet still mountain based, fresh) water?